It is well-known that structured surface materials can increase the apparent on axis brightness of Lambertian backlights such as the ones commonly used with liquid crystal displays (LCD's). These films are commonly known as brightness enhancement films. Such films function by refracting light that would be emitted at large angles relative to the display axis into smaller angles. Light that would, absent the brightness enhancement film, be emitted at a small angle relative to the display axis is reflected back into the backlight for recycling. There it strikes a diffuse reflector, randomizing its direction. After being reflected it will reenter the brightness enhancement film and be refracted or reflected as before.
The effect of brightness enhancement film is to increase the amount of light emitted at small angles to the axis of the display by reducing the amount emitted at greater angles. Thus, as one looks at a display at increasing angles with respect to the axis, the perceived brightness will decline. A typical film will provide a slowly declining brightness from a direction parallel to the axis to an angle of about 35 degrees to the axis. Between 35 and 40 degrees the perceived brightness will decline very rapidly. This effect is known as a sharp cutoff. Some users would prefer a more gradual or soft cutoff.